1. Field of the Invention
The present invention relates to a magnetic disc and a magnetic disc substrate.
2. Discussion of Background
A magnetic recording medium (hereafter referred to simply as a medium) having one or multiple undercoat layers, a continuous thin film magnetic layer or continuous thin film multiple magnetic layers, a protective layer and a lubricant layer sequentially laminated on a non-magnetic substrate of a disc shape, and a magnetic head for recording and reproduction (hereafter referred to simply as a head) constitutes the main components of a hard disc drive apparatus. At the initiation of the operation, the head and the medium are in contact with each other. During rotation of the medium, the head is lifted from the surface of the medium by the resistance of air. At the termination of the operation, the head will again be in contact with the medium. This system is called a contact-start-stop (CSS) system. If the surface of the medium is smooth, the head is likely to stick to the medium (this phenomenon is called "head stick"), which is likely to cause head crash or hinder the initiation of the hard disc drive apparatus.
It is known to prevent such head stick by preliminarily forming roughhess on the surface of the medium by various methods such as a mechanical method, an etching method (such as the one disclosed in Japanese Unexamined Patent Publication No. 37718/1989) or a deposit method, to reduce the friction coefficient between the head and the medium.
On the other hand, to increase the recording density of a hard disc drive apparatus, it is necessary to reduce the guaranteed minimum flying height (so-called glide height) of the magnetic head. To reduce the glide height, it is known to control the maximum roughness (Rmax) or the average roughness (Ra) of the surface roughness of the magnetic disc within a certain range. However, with such conventional techniques, it has been difficult to fully satisfy the above-mentioned two requirements. Namely, the medium which satisfies the CSS durability without causing head stick, tends to hardly satisfy the glide height requirement. On the other hand, the one which satisfies the glide height requirement tends to hardly satisfy the CSS durability.